Abstract
Abundant fossils from the major animal phyla appear rather suddenly in Cambrian deposits, starting 544 million years ago. Many paleontologists hold that this fossil “explosion” reflects that most animal phyla originated in the late Neoproterozoic, during the 100-150 million years preceding the Cambrian. Others claim that their origin is as much as twice as old, dating to more than one billion years ago. Our analysis of 18 protein-encoding genes yields estimates of 602-736 million years ago for the divergence between protostomes (annelids, arthropods, and mollusks) and deuterostomes (echinoderms and chordates). Estimates of the divergence between echinoderms and chordates range 560-628 million years ago. These results support a late Neoproterozoic origin of the major animal phyla and are consistent with the recent discovery of 570 million-years-old spongae and animal embryos.
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